The present invention relates to inert anodes for dissipation of continuous current.
Metal structures, especially oil, gas and water pipelines as well as water and gas distribution networks buried or immersed in sea water or the like are subjected to spontaneous corrosion or corrosion caused by stray currents.
In order to prevent damage by these destructive phenomena, cathodic protection plants are provided. An indispensible component of these plants is a ground-bed formed by one or more anodes. The number of the anodes depends on their characteristics, the current to be dispersed and the expected working duration such a ground-bed has to have.
Initially the ground-beds were formed by using pieces of rail, pipes and other pieces of scrap iron as anodes. Because of high consumption rates (10 kg/A per annum), these types of anodes were subsequently substituted by graphite or silicon-iron anodes. These anodes usually have a cylindrical shape and low consumption rates (approximately 1 kg/A per annum).
These second types of anode are called "semi-inert" in virtue of their extensive life span. Recently, in addition to the above mentioned types other types of anodes have been introduced with an extremely extensive life span. They have therefore been termed "indissoluble" or "inert" anodes. These anodes are composed of titanium laminars or profiles coated over by a thin layer of indissoluble platinum produced by electrolytic means or by thermally produced oxides, especially of titanium iridium or ruthenium.
Even though, the latter types of anodes are widely used in industrial electrolytic plants, they did not find a practical application as part of the above-mentioned cathodic protection plants. Due to the low conductivity of the laying ground, the cathodic protection plant ground beds and therefore their anodes have to have an extensive dissipation surface, thus involving a considerable mass. Since titanium is a precious metal and therefore costly, the anodes of titanium (rods, tubes or profiles), are generally limited to a 2 centimeter diameter. In order to obtain a dissipating surface equal to that obtained when using a ground-bed of graphite or silicon-iron anodes of an approximately 8 centimeter diameter, titanium anodes of the same length and four times greater in number have to be used. This makes the cost of ground-beds of titanium anodes uneconomical. Since the required thickness of the titanium support useful for the application, efficiency and functioning of the dissipating layer needs only be of a few microns, it is self-evident that the use of titanium rods and tubes to serve as traditional anodes creates and unnecessary waste of valuable material.